Single layer fuel lines and vapor return lines of synthetic materials such as polyamides have been proposed and employed in the past. Fuel lines employing such materials generally have lengths of at least several meters. It is important that the line, once installed, not materially change during the length of operation, either by shrinkage or elongation or as a result of the stresses to which the line may be subject during use.
It is also becoming increasingly important that the lines employed be essentially impervious to hydrocarbon emissions due to permeation through the tubing. It is anticipated that future Federal and state regulations will fix the limit for permissible hydrocarbon emissions due to permeation through such lines. Regulations which will be enacted in states such as California will fix the total passive hydrocarbon emission for a vehicle at 2 g/m.sup.2 per 24 hour period as calculated by evaporative emission testing methods such as those outlined in Title 13 of the California Code of Regulations, section 1976, proposed amendment of Sep. 26, 1991. To achieve the desired total vehicle emission levels, a hydrocarbon permeation level for the lines equal to or below 0.5 g/m.sup.2 per 24 hour period would be required.
Finally, it is also imperative that the fuel line employed be impervious to interaction with corrosive materials present in the fuel such as oxidative agents and surfactants as well as additives such as ethanol and methanol.
Various types of tubing have been proposed to address these concerns. In general, the most successful of these have been co-extruded multi-layer tubing which employ a relatively thick outer layer composed of a material resistant to the exterior environment. The innermost layer is thinner and is composed of a material which is chosen for its ability to block diffusion of materials such as aliphatic hydrocarbons, alcohols and other materials present in fuel blends, to the outer layer. The materials of choice for the inner layer are polyamides such as Nylon 6, Nylon 6.6, Nylon 11, and Nylon 12.
Alcohol and aromatic compounds in the fluid conveyed through the tube diffuse at different rates through the tubing wall from the aliphatic components. The resulting change in the composition of the liquid in the tubing can change the solubility thresholds of the material so as, for example, to be able to crystalize monomers and oligomers of materials such as Nylon 11 and Nylon 12 into the liquid. The presence of copper ions, which can be picked up from the fuel pump, accelerates this crystallization. The crystallized precipitate can block filters and fuel injectors and collect to limit travel of the fuel-pump or carburetor float as well as build up on critical control surfaces of the fuel pump.
In U.S. Pat. No. 5,076,329 to Brunnhofer, a five-layer non-corrugated fuel line is proposed which is composed of a thick corrosion-resistant outer layer formed of a material known to be durable and resistant to environmental degradation such as Nylon 11 or Nylon 12. The tubing disclosed in this reference also includes a thick intermediate layer composed of conventional Nylon 6. The outer and intermediate layers are bonded together by a thin intermediate bonding layer composed of a polyethylene or a polypropylene having active side chains of maleic acid anhydride. A thin inner layer of aftercondensed Nylon 6 with a low monomer content is employed as the innermost region of the tubing. The use of Nylon 6 as the material in the inner fluid contacting surface is designed to eliminate at least a portion of the monomer and oligomer dissolution which would occur with Nylon 11 or Nylon 12. The thin innermost layer is bonded to the thick intermediate layer by a solvent blocking layer formed of a copolymer of ethylene and vinyl alcohol with an ethylene content between about 30% and about 45% by weight. The use of a five layer system was mandated in order to obtain a tubing with the impact resistance of Nylon 12 and the low monomer/oligomer dissolution of Nylon 6. It was felt that these characteristics could not be obtained in a tubing of less than five layers.
In U.S. Pat. No. 5,038,833 also to Brunnhofer, a three-layer non-corrugated fuel line without the resistance to monomer/oligomer dissolution is proposed in which a tube is formed having a co-extruded outer wall of Nylon 11 or Nylon 12, an intermediate alcohol barrier wall formed from an ethylene-vinyl alcohol copolymer, and an inner water-blocking wall formed from a polyamide such as Nylon 11 or Nylon 12. In DE 40 06 870, a fuel line is proposed in which an intermediate solvent barrier layer is formed of unmodified Nylon 6.6 either separately or in combination with blends of polyamide elastomers. The internal layer is also composed of polyamides, preferably modified or unmodified Nylon 6. The outer layer is composed of either Nylon 6 or Nylon 12.
Another non-corrugated tubing designed to be resistant to alcoholic media is disclosed in UK Application Number 2 204 376 A in which a tube is produced which has an thick outer layer composed of 11 or 12 block polyamides such as Nylon 11 or Nylon 12 which may be used alone or combined with 6 carbon block polyamides such as Nylon 6 or 6.6 Nylon. The outer layer may be co-extruded with an inner layer made from alcohol-resistant polyolefin co-polymer such as a co-polymer of propylene and maleic acid.
Heretofore it has been extremely difficult to obtain satisfactory lamination characteristics between dissimilar polymer layers. Thus all of the multi-layer tubing proposed previously has employed polyamide-based materials in most or all of the multiple layers. While many more effective solvent-resistant chemicals exists, their use in this area is limited due to limited elongation properties, strength and compatibility with materials such as Nylon 11 and 12.
It has also been difficult to obtain a multi-layer tubing which is capable of bending or being bent to conform to the contours in the particular automotive vehicle. In most automotive applications, the tubing employed must be capable of bending to a variety of angles both obtuse and acute to conform to the layout and the space requirements in the specific vehicle design. Materials such as conventional Nylons possess a significant elastic memory which makes it difficult to successfully bend a piece of tubing into the shape or contours necessary in the particular automotive application. Conventional non-corrugated tubing when bent will experience significant reduction in its useful life due to fatigue and stress at or near the contour sight.
In order to obviate this problem, it has been proposed that the tubing be corrugated at the appropriate contour regions to accommodate the particular bends and angles without imparting under fatigue or stress. The contour region may include a plurality of annularly oriented accordion-like pleats which permit one longitudinal side of the pleats to be compressed in on themselves while the opposing longitudinal side of the annular pleats can be extended outwardly from one another to accommodate the necessary bend.
In the past, single layer corrugated tubing has been produced. To produce tubing with suitable regions of corrugation or other suitable annular contours, a suitable polymeric monofilament is extruded from a suitable extrusion head preferably in a semi-molten state. The material is, then, introduced into a suitable die means where it is forced to conform with the contours of the interior surface of the die. The semi-molten material is allowed to completely solidify thereby producing tubing material with the suitable corrugated ridges.
This process is reasonably successful for producing single-layer polymeric tubing materials having wall thicknesses between about 0.75 mm and about 2.0 mm. Materials such as Nylon 11 and Nylon 12 have inherent characteristics which permit these materials to stretch to conform to the ridges and contours present in the molding dies upon extrusion without compromising the integrity of the tubing wall. However, when wall thicknesses of materials such as Nylon 11 and Nylon 12 are decreased below this level, the elastic expansion capacity of the extruded polymeric material is exceeded. This results in non-homogeneous wall thickness in the corrugation regions. Thus, current methods for producing corrugated tubing are limited to the production of single-layer tube having a wall thickness above 0.75 mm.
Additional problems are presented when the tubing material employed is made of layers of differing chemical compositions. Different polymeric materials exhibit different degrees of elasticity, stretch and the like. When multiple layer tubing is expanded to conform to the contours of the inner surface of the molding die, the differing expansion characteristics can cause the various layers to delaminate, destroying the advantages sought by the use of multi-layer tubing. Additionally, in order to obtain a suitable total wall thickness, each of the various layers extruded in a multi-layer tube can have thicknesses which approach or are below the thickness necessary to accommodate stretch due to deformation. The post-extrusion molding process can result in localized areas within the corrugated region of the tubing in which the integrity of the various layers is severely compromised. It is possible that localized areas within the corrugation would be characterized by a complete absence of one of the layers due to this unequal deformation.
It would be highly desirable from the standpoint of economics and ease of manufacturing to provide a method by which corrugated multi-layer tubing could be produced as part of a coextrusion process in a continuous manner.
It would also be desirable to provide a multi-layer tubing material which could be employed in motor vehicles which would be durable and prevent or reduce permeation of organic materials therethrough. It would also be desirable to provide a tubing material which would be essentially nonreactive with components of the liquid being conveyed therein. It would also be desirable to provide a multi-layer tubing material which exhibits these characteristics which has localized or overall areas of corrugation.